This invention relates to a fluorescent lamp assembly for use in an image scanner which reads out an image as an electric signal.
Conventionally, an image scanner is known, which scans an original to convert an image thereon to an electric signal. The image scanner comprises a scanning head having a line sensor in which a plurality of picture elements are arranged in a single row. In use, the image scanner or the original moves in one direction to cause the scanning head to carry out a main scanning of the original. The image scanner or the original also moves perpendicular to the main scanning direction to cause the scanning head to carry out an auxiliary scanning of the original. The image scanner converts the light quantity incident upon an image sensor to an electric signal corresponding to the incident light quantity, thereby reading out the image on the original.
The scanning head is composed of a light source, an objective lens, a mirror and the sensor, all of which extend over the entire length of the scanning head in the main scanning direction. The main scanning by the image sensor is such that a portion of the original illuminated with light from the light source is imaged onto the light receiving surface of the image sensor by the objective lens.
A fluorescent lamp which is generally a discharge lamp, is employed as the light source of the scanning head described above.
The fluorescent lamp has its luminescent characteristic such that it has a luminous intensity distribution in the longitudinal direction and an absolute value of a light quantity. As shown in FIG. 4, the luminous characteristic largely varies during a period from the initial stage of light emission after having been turned on, to a stable stage. After a certain time (of the order of 30 seconds, for example) has elapsed after having been turned on, heat generation due to the light emission raises the temperature of the glass tube of the fluorescent lamp to a certain degree, and the luminous characteristic is stabilized.
In the image scanner, the fluorescent lamp emits light simultaneously with the start-up of scanning by the image scanner, and is turned off after having carried out the main and auxiliary scannings. Thus, the image scanner always effects scanning just at the unstable stage of the luminescent characteristic. As a result, an accurate read-out signal cannot be obtained.
In the meantime, the luminous intensity of the fluorescent lamp varies depending upon the longitudinal position on the lamp, even during the stable light emission period. If the luminous intensity distribution is stable, however, it is possible to correct the readout signal. It has been impossible, however, to correct the instability of the luminescent characteristic as mentioned above.